tobes wrote:But to infer from this that that the there is nothing outside the objects of our consciousness is quite another step. You deploy an argument about causality to underpin that: but how can your account of causality be sustained if it nothing can be established external to consciousness?
Just to add to what Dexing has already said: The two reasonings already mentioned are (1) invariable co-observation (sahopalambhaniyama; lhan cig dmigs par nges pa) and (2) invariable sameness of appearances and mind as the nature of mere lucidity (snang ba dang sems gsal tsam gyi ngo bor gcig par nges pa).
The analysis is not complete until we also investigate if gross or subtle forms can be established as single entities. Gross appearances, such as the visible appearance of a house, are not single entities because they are comprised of various aspects (variegated hues, tones, shapes). And on a subtle level, particles are not single entities because they have directional parts and differing characteristics in relation to other particles. If a particle were truly dimensionless and of a unitary characteristic, then it wouldn't be possible to aggregate many such particles into three dimensional gross appearances. Therefore, no single, unitary particle can be established. And if a single particle cannot be established, then many such particles cannot be established. Appearances are merely aspects of cognition. This is an example of the mādhyamaka reasoning of neither one nor many.
These reasonings are given in Kamalaśīla's Tattvasaṃgrahapanjika, the commentary to Śāntarakṣita's Tattvasaṃgraha. Of course, for these reasonings to be effective in countering our habitual sense of externality, they have to be engaged repeatedly in meditative equipoise. Sustained Mahāyāna and Vajrayāna faith, as well as 15 - 20 years of mahāmudrā practice doesn't hurt either.
All the best,